Light-emitting diodes (LEDs) are now being used for direct LED displays (where groupings of LEDs essentially comprise a pixel and are used to generate a large image of LED light) as well as the backlight unit for liquid crystal displays (LCDs). Modern displays have become increasingly brighter, with some LCD backlights producing 800-1,500 nits or more. Sometimes, these illumination levels are necessary because the display is being used outdoors, or in other relatively bright areas where the display illumination must compete with other ambient light. In order to produce this level of brightness, LEDs (whether used for backlighting purposes or for direct LED displays) may produce a relatively large amount of heat. Further, displays of the past were primarily designed for operation near room temperature. However, it is now desirable to have displays which are capable of withstanding large surrounding environmental temperature variations. For example, some displays are capable of operating at temperatures as low as −22 F and as high as 113 F or higher. When surrounding temperatures rise, the cooling of the display components can become even more difficult.
Still further, in some situations radiative heat transfer from the sun through a front display surface can also become a source of heat. In some locations 200 Watts or more through such a front display surface is common. Furthermore, the market is demanding larger screen sizes for displays. With increased electronic display screen size and corresponding front display surfaces, more heat will be generated and more heat will be transmitted into the displays.
LED efficiency is typically characterized by a unit of luminance per a unit of power. Sometimes, this is characterized as lumens per Watt (lumens/W). It has been observed, that LED efficiency typically decreases as the temperature of the LED increases. Thus, the hotter an LED gets, the less light is generated per the same amount of power input. In some LED assemblies, there can be substantial temperature variation across the assembly where some areas are cool while others are hot. This is especially seen in large LED assemblies which are exposed to warm ambient temperatures and/or sunlight exposure. Thus, when regions of the LED assembly are warmer than others (‘hot spots’) the LEDs within these regions will have their luminance affected. To an observer of the display, this variation in luminance can be viewed as non-uniformity across the display. This non-uniformity is undesirable as it can affect the image quality.